Abstract

Abstract. Smoke from biomass and peat burning has a notable impact on ambient air quality and climate in the Southeast Asia (SEA) region. We modeled a large fire-induced haze episode in 2006 stemming mostly from Indonesia using the Weather Research and Forecasting model coupled with chemistry (WRF-Chem). We focused on the evolution of the fire plume composition and its interaction with the urbanized area of the city state of Singapore, and on comparisons of modeled and measured aerosol and carbon monoxide (CO) concentrations. Two simulations were run with WRF-Chem using the complex volatility basis set (VBS) scheme to reproduce primary and secondary aerosol evolution and concentration. The first simulation referred to as WRF-FIRE included anthropogenic, biogenic and biomass burning emissions from the Global Fire Emissions Database (GFED3) while the second simulation referred to as WRF-NOFIRE was run without emissions from biomass burning. To test model performance, we used three independent data sets for comparison including airborne measurements of particulate matter (PM) with a diameter of 10 μm or less (PM10) in Singapore, CO measurements in Sumatra, and aerosol optical depth (AOD) column observations from four satellite-based sensors. We found reasonable agreement between the model runs and both ground-based measurements of CO and PM10. The comparison with AOD was less favorable and indicated the model underestimated AOD, although the degree of mismatch varied between different satellite data sets. During our study period, forest and peat fires in Sumatra were the main cause of enhanced aerosol concentrations from regional transport over Singapore. Analysis of the biomass burning plume showed high concentrations of primary organic aerosols (POA) with values up to 600 μg m−3 over the fire locations. The concentration of POA remained quite stable within the plume between the main burning region and Singapore while the secondary organic aerosol (SOA) concentration slightly increased. However, the absolute concentrations of SOA (up to 20 μg m−3) were much lower than those from POA, indicating a minor role of SOA in these biomass burning plumes. Our results show that about 21% of the total mass loading of ambient PM10 during the July–October study period in Singapore was due to biomass and peat burning in Sumatra, but this contribution increased during high burning periods. In total, our model results indicated that during 35 days aerosol concentrations in Singapore were above the threshold of 50 μg m−3 day−1 indicating poor air quality. During 17 days this was due to fires, based on the difference between the simulations with and without fires. Local pollution in combination with recirculation of air masses was probably the main cause of poor air quality during the other 18 days, although fires from Sumatra and probably also from Kalimantan (Indonesian part of the island of Borneo) added to the enhanced PM10 concentrations. The model versus measurement comparisons highlighted that for our study period and region the GFED3 biomass burning aerosol emissions were more in line with observations than found in other studies. This indicates that care should be taken when using AOD to constrain emissions or estimate ground-level air quality. This study also shows the need for relatively high resolution modeling to accurately reproduce the advection of air masses necessary to quantify the impacts and feedbacks on regional air quality.

Highlights

  • Biomass burning plays an important role in atmospheric composition and chemistry (Crutzen and Andreae, 1990; Lamarque et al, 2010)

  • It is important to note here that our increase of 28 % is substantially lower than Petrenko et al (2012) who showed an underestimation up to 300 % of biomass burning aerosol emissions in Indonesia, or in Marlier et al (2013) who increased the aerosol emissions from fires with 226 %

  • The comparison with observations of PM10 and carbon monoxide (CO) showed that the WRF-Chem model managed to reproduce quite accurately the surface concentrations

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Summary

Introduction

Biomass burning plays an important role in atmospheric composition and chemistry (Crutzen and Andreae, 1990; Lamarque et al, 2010). Fire activity is highly modulated by the El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) (Hong et al, 2008; Field et al, 2009; Reid et al, 2013) Populated areas such as Java and the city of Singapore are located relatively close to large fires mainly in Sumatra and Kalimantan and regularly show high particulate pollution levels which are often related to emissions from forest, agriculture and peat fires (Hyer and Chew, 2010; Salinas et al, 2013a, b; Wang et al, 2013). Air pollution caused by aerosol particles is of concern because of reduction in visibility and adverse environmental and health impacts (Mauderly and Chow, 2008) Depending on their size and chemical composition, aerosol particles can penetrate into the respiratory system and increase throat and lung infections (Karthikeyan et al, 2006; Pavagadhi et al, 2013). We used WRF-Chem to (1) advect the aerosol and gaseous precursor concentrations

Model setup
Comparison with observations
Aerosol plume analyses: composition and distribution
28 Sep–13 Oct WRF-FIRE WRF-NOFIRE
Findings
Conclusions

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